The present invention is directed toward numerous features within a drive train of a type having a transmission unit which is capable of providing a generally continuous positive coupling within the drive train. Such a transmission unit is preferably embodied within a hydrostatic unit including at least one hydraulic translating means such as a pump or motor capable of variable displacement. Again, it is preferable that both the pump and motor be capable of variable displacement.
The present invention is also directed toward broadening the operating capabilities of such a transmission unit by combining it in series with a multiple speed range transmission.
The present invention is particularly concerned with automatically synchronizing operation of the two transmission units to achieve a smooth transition of torque transmitting capacity and operating speeds for the drive train.
The present invention is also concerned with providing automatic speed controls for a transmission unit of the type first noted above. Preferably, the means for synchronizing operation of the two transmission units as well as means for accomplishing the speed control functions referred to above are embodied in hydraulic controls as described in greater detail below. However, it will be apparent from the following description that the same or similar functions can be achieved through other control elements such as electronic control circuits.
Substantial efforts have been expended and are still being undertaken in an attempt to more effectively use the numerous advantages afforded by hydrostatic transmissions. Generally, hydrostatic transmissions present special problems in control since displacement of both the pump and motor must be varied in exact sequence in order to achieve efficient operation and to provide suitable regulation over torque transmitting capacity and operating speed of the drive train. For example, in such a hydrostatic transmission, the pump is commonly set at zero displacement with the motor being at or near its maximum displacement when the drive train is in a neutral condition.
For acceleration of the vehicle, displacement of the pump may first be varied toward a maximum value while the motor remains at its maximum displacement in order to develop maximum torque transmitting capacity for initially accelerating the vehicle. After the pump reaches maximum displacement, displacement of the motor may be gradually reduced to further accelerate the vehicle.
Usually, as the motor approaches minimum displacement, the full operating range of the hydrostatic transmission is realized according to the presently available prior art unless the transmission includes relatively sophisticated developments such as multiple pumps for extending the torque transmitting capacity of the hydrostatic transmission. However, such solutions tend to make the transmissions very complex while even further increasing difficulties in properly sequencing operation of the variable displacement components therein.
Accordingly, it is desirable to provide a relatively simple and easily controlled means for expanding the torque transmitting capacity of a hydrostatic transmission unit in order to better adapt hydrostatic transmissions for use in a wide variety of vehicles. In particular, hydrostatic transmissions with expanded torque transmitting capacity would be useful in material handling machines such as earth moving vehicles where a single prime mover is employed both to supply motive power for the vehicle as well as to operate one or more implements which may also have substantial instantaneous power requirements relative to the maximum output capability of the prime mover.
An improved hydrostatic transmission would be particularly useful in such machinery for numerous reasons. For example, material handling vehicles must be adapted both for transport operation at relatively high speeds as well as low speed, high torque operation of the vehicle together with intermittent operation of its implements. At such times, the vehicle may be subjected to frequent changes of direction and continuous accelerating and/or decelerating operation. A hydrostatic transmission unit is very suitable for such applications particularly if automatic controls are provided to maximize use of the available power from the single prime mover.
A hydrostatic transmission could also be adapted for relieving the engine and increasing output torque during lug conditions by selectively and automatically reducing vehicle speed. In addition, a hydrostatic transmission would enable available power from a prime mover to be more precisely proportioned between what is required for motive power to the vehicle as well as supplying preferential power requirements of various implements mounted on or associated with the vehicle.
Examples of presently available hydrostatic transmissions for use in such vehicles are set forth, for example, in U.S. Pat. No. 3,302,390 to Christenson and U.S. Pat. No. 3,477,225 to Cryder et al, the last noted patent being assigned to the assignee of the present invention. The Christenson patent discloses a transmission which is adapted for operation of track-type vehicles whereas the present invention is particularly intended for use with wheeled vehicles since it provides only a single primary drive train. However, it will be apparent that numerous features of the present invention could also be used, for example, with track-type vehicles including dual primary drive trains.
Other examples of prior art in the area of hydrostatic transmissions include U.S. Pat. Nos. 3,187,509; 3,212,263; 3,236,049; 3,238,724; 3,247,669; 3,273,344; 3,285,000; 3,324,797; 3,331,480 and 3,411,297.